Muwatta Malik, Book 54, Hadith 36 (USC-MSA) ⓘ
Malik related to me from Abd ar-Rahman ibn Harmala that Said ibn al-Musayyab heard the Messenger of Allah, may Allah bless him and grant him peace, say, "Shaytan concerns himself with one and two. When there are three, he does not concern himself with them."
وَحَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ حَرْمَلَةَ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ كَانَ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الشَّيْطَانُ يَهُمُّ بِالْوَاحِدِ وَالاِثْنَيْنِ فَإِذَا كَانُوا ثَلاَثَةً لَمْ يَهُمَّ بِهِمْ " .
also referenced under: Book 54, Hadith 1802 (In-Book)